The present invention relates to a process for preparing graft copolymers based on styrene and an EPDM elastomer.
Polystyrene is a thermoplastic which is widely used in the plastics industry by virtue of the ease with which it can be obtained and worked. Unfortunately, its use is limited as a result of its lack of heat stability and of its mediocre impact strength.
In order to improve these properties, it has been proposed to use copolymers of styrene with other monomers such as acrylonitrile or acrylates. It has also been proposed to use graft polymers obtained by polymerizing styrene with an elastomer, e.g., a predominantly saturated elastomer such as an ethylene/propylene elastomer or an ethylene/propylene/polyene monomer elastomer (EPDM rubber).
However, the use of such graft copolymers presents numerous difficulties. In particular, the predominantly saturated rubbers, which have a low proportion of double bonds, do not favor the grafting of styrene onto the elastomer. (The terms "rubber" and "elastomer" are used interchangeably herein.) Accordingly, the compositions obtained have a very low grafting ratio, i.e., a very low proportion of polystyrene grafted onto the elastomer, which is at most of the order of 0.5% by weight.
Furthermore, microscopic observation makes it possible to establish that the resultant compositions contain large globules of rubber which are dispersed in the composition without any cohesion, and the size of which can be as large as 10 to 15 microns.
The composition obtained with a dispersion of this type does not have a fine microstructure, resulting in products which are not very shiny and whose surface appearance is unsatisfactory. Apart from the problems described above, which relate to the heat stability and resistance to aging, the impact strength, the grafting ratio of polystyrene and also the microstructure, it is necessary, in order to obtain compositions possessing advantageous properties, to increase the degree of crosslinking of the elastomer.
When styrene is polymerized in the presence of elastomers of the ethylene/propylene/polyene monomer type, e.g., EPDM rubbers, crude graft products are obtained which comprise, in particular, a graft copolymer formed by reaction of the rubber with the styrene monomer, and rubbery units consisting of "bridged" units, i.e., rubber units joined to one another by chemical bonds. The crosslinking density of the "bridged" rubber is expressed as the "swelling index", which is inversely proportional to the degree of crosslinking, i.e., the lower the swelling index the higher the crosslinking density and the more advantageous the properties of the copolymer.
To solve these problems, processes have been proposed which include sophisticated reaction steps of the type involving the use of a mixture of special solvents, partial oxidation of the rubber before grafting, or multiple-stage polymerization. Not all these processes are satisfactory and they lead, in particular, to graft polymers which frequently still have a low grafting ratio together with a mediocre degree of crosslinking and a coarse microstructure. Examples of such processes are found, e.g., in French Pat. Nos. 2,320,950; 2,263,260; and 2,164,832.
The process of the present invention makes it possible to obtain graft copolymers not having these disadvantages.